drm/i915/fixed: simplify is_fixed16_zero()

Simply return the condition. No functional changes.

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181116120729.7580-4-jani.nikula@intel.com